Fluid circulation system for a oncethrough type steam generator



22, 1954 c. STROHMEYER, JR 3,162,179

FLUID CIRCULATION SYSTEM FOR A ONCE-THROUGH TYPE STEAM GENERATOR Flled Dec. 5, 1962 6 Sheets-Sheet 1 STEAM GENERATOR I 42 I 2 5G 5 6O 3 52 53 6 6G I 60. I I 136' i 27 46- FLASH 5a TURBINE TANK GENERATOR r 7 DEAERATOR 22 39 9 INVEN TOR.

Charles Slrohmeyer, Jr.

his ATTORNEY 3, 1 62 1 ONCE-THROUGH 6 Sheets-Sheet 2 1964 c. STROHMEYER, JR

FLUID CIRCULATION SYSTEM FOR A TYPE STEAM GENERATOR Filed Dec. 5, 1962 INVENTOR. Charles S frohmeyer, Jr. IBY

v 823 N n/@ owns N Nv \l his ATTORNEY v LWN Q C 13 1964 c. STROHMEYER, JR 3,162,179

mun CIRCULATION SYSTEM FOR A ONCE-THROUGH TYPE STEAM GENERATOR 6 Sheets-Sheet 3 Filed Dec. 5, 1962 STEAM CHARACTERISTICS SSOOPSIA mifi m h A a 3 I r i W. 00000 w w m w w 500 600 700 800 900 I000 IIOO I200 I300 I400 I500 ENTHALPY, BTU/LB.

om wm PEmwmEm STEAM GENERATOR TURBINE-GENERATOR COND. PUMP ,NVENTOR Charles Strohmeyor, Jr. a I

X his ATTO NEY DEAERATOR Fig. 4.

FEEDWATER PUMP 1964 c. STROHMEYER, JR 3,162,179

FLUID CIRCULATION SYSTEM FOR A 0NCE-THROUGH TYPE STEAM GENERATOR Filed Dec. 5. 1962 6 Sheets-Sheet 5 ig5b. a an 3' 3 i 5 3" 3a. 3' 3a INVENTOR.

Charles Strohmcycr Jr.

' ATTORNEY Fig.6.

1954 c. STROHMEYER, JR 3,162,179

FLUID CIRCULATION SYSTEM FOR A ONCE-THROUGH TYPE STEAM GENERATOR 6 Sheets-Sheet 6 Filed Dec. 5, 1962 INVENTOR. (.har|es Strohmeyer, Jr.

his ATTORNEY United States Patent Ofifice 3,162,179 Patented Dec. 22, 1964 3,162,179 FLUID CIRCULATION SYSTEM FOR A QNCE- THROUGH TYPE STEAM GENERATOR Charles Strohmeyer, In, Wyornissing, Pan, assignor to Gilbert Associates, Inc., Reading, Pa. Filed Dec. 5, 1962, Ser. No. 242,395 6 Ciaims. (Cl. 122--4(l6) V fluid flow through the generating section may be arranged serially or parallelly, or in combination thereof, through the generating section component conduit groups. in a way to more nearly equalize mass flow of fluid in the individual conduits of each group throughout the load range of the steam generator.

A more specific object of this invention is to provide a control means for transferring from series to parallel flow through the generating section component conduit groups when increasing steam generator load, forming combinations therebetween gradually and uniformly so that flow will be properly proportioned in all generating sect-ion conduits without abrupt or cyclic changes in flow and to reverse the operation when decreasing steam generator load.

A further specific object of this invention is to provide a pumping and flow control system in combination with directional flow control and throttling means in the generating section and connecting conduits to accomplish the above objective.

A further specific object of this invention is to provide a means whereby the generating section component conduit groups may be operated in series arrangement during startup and low load operation, employing directional flow controlled conduits between the component conduit groups, in conjunction with positive isolation betwee. the upstream parallel feed from the preheating section to each component conduit group except for the first component conduit group in the series ararngement and in conjunction with positive isolation between each component conduit group and the parallel downstream discharge to the superheating section except for the last component conduit group in the series arrangement.

Other objects and advantages of the present invention Will become more apparent from a study of the following description taken with the accompanying drawings wherein:

FIG. 1 is a diagrammatic arrangement of a once-through type steam generator having means to control the transition from series to parallel fluid flow through two generating section component conduit groups in combination with a flow cycle including a steam turbine driver receiving steam from'the steam generator super-heater, a condenser receiving exhaust steam from the turbine driver and means to return the condensed fluid to the steam generator, preheater, in accordance with the present invention;

FIG. 1a is a control detail for the system illustrated in FIG. 1;

FIG. 2 is a modification of the steam generator shown in FIG. 1 illustrating how fluid flow through multiple generating section component conduit groups may be controlled in parallel or series ararngement, or combinations therebetween;

FIG. 2a is a control detail for the system illustrated in FIG 2;

FIG. 3 is a chart or plot of water-steam fluid characteristics at 3500 p.s.i.a. and shows fluid temperature and fluid specific volume plotted against fluid enthalpy (B.t.u./-lb.);

FIG. 4 is a modification of the system illustrated in FIG. 1 which minimizes temperature differentials between generating section individual conduit elements when they are operated in series arrangement;

FIG. 5 shows a physical diagrammatic arrangement of the system illustrated in FIG. 4 as applied to a reheat unit and shows the arrangement of the furnace water wall structure (a control detail is also illustrated);

FIGS. 5a, b, c, d, e and 7 illustrate details of the furnace waterwall shown in FIG. 5;

FIG. 6 is a modification of the system illustrated in FIGS. 2 and 4; and,

FIG. 7 is a modification of the system illustrated in FIG. 4.

General Description of Present Invention As the output steam fiow from a once-through type steam generator is decreased, mass flow in the individual conduits i decreased proportionately. It follows that pressure drop through the conduits decreases exponentially. For example, pressure drop at one half load is approximately one-fourth what it was at full load. Flow through the individual conduits is dependent on available pressure drop across the conduits. Therefore, when pressure drops are low, uniformity and balance of flow among or between conduits becomes less stable.

Once-through type of steam generators must be designed for the minimum flow condition. A pressure drop is selected which will give flow stability under conditions of operation at this time, taking into account unbalances in heat absorption. If the minimum flow is low, say at one-third of maximum rating, then the pressure drop through the controlling circuits at rated flow will be approximately nine times What it is at one-third flow. Therefore, pressure drop through once-through boilers at maximum rating tends to be high and wasteful of boiler feed pump power.

Minimum mass flow as a percent of design also determines the margin of safety required for pressure drop during startup. With higher mass flow and less temperature rise through the individual circuits, less pressure drop margin is required. If the minimum flow occurs when the individual conduits are all operated in parallel and the total minimum flow is high to reduce pressure drop at rated load, then during startup before the steam generator is up to operating temperature and pressure and supplying superheated steam to a user, or the reverse for shutdown and cooling, an excessive amount of fluid must be handled by a startup conduit and heat manipulation system bypassing the normal steam consumer. Such a system is expensive and is often wasteful of heat when first transferring the steam generator output from the bypass system to a consumer.

In a steam generator having a combustion furnace and hot gas flow path for providing heat input to the heat absorption conduits, the heat release in the combustion zone can cause the greatest fluid flow unbalance among heat absorption conduits in this zone. When fluid flows are minimum with respect to rated flow, fluid flow unbalance can result in overheating and damagingthe heat absorption conduits. sorption conduits normally receive heat from and are immediately adjacent the furnace combustion zone. Therefore, these conduits need the greatest design pro- The generating section heat ab li e a ev V lfThejproblem of alternate series and parallel operation of the generating section conduit groups relatesto tran- 10!! rom on m dwf gperation th O h tti d d ring the intermediate mode. In parallel operation, the pres 0 sure drop through all of the;generatingsectionjcompo crating section componentconduit groups'would produce the same mass flow in each group'as all the minimum flow passed through the same groups when arranged in parallel. jAlternately, if the flow quantity to the steam generation section remained the'same as for the parallel. V

the individual.

arrangement, the minimum flowthrough conduits for the series arrangement would be twice that normally flowing through the same circuits for the parallel arrangement. By increasing minimum flow in the individual conduits, pressure drop at full load can be reduced substantially; for example, a design for a generating section having a minimumflow of SOpercent. of rated flow inthe individual tube conduits produces 'a pressure drop at .rated flow which is lessthan half of that'which results where the generating section individual tube can duits are designed for a minimum flow of 33 percent of rated flow.

Where the generating section'is operated in two series groups, .each having a minimum flow of 50 percent of rated flow converting to parallel group arrangement'at rated flow, the minimurnfeedwater flow to the steam gen-" erator would be only 25 percent of rated flow. In the' series arrangement, flow can be mixed thoroughly between the groups and there is less temperature and specific volume rise in each group; The series arrangemcntincreases the number of fluid mixing'points at a time when they are needed. The greater minimum mass flow through the individual generating sectionfc'onduits in conjunction with less change in the fluid specific volume between'rnixing section, customarily operated with paraladditive with respect to the total pressure drop through the generatingsection. The pressure drops through the individual groups will not be equal because of fluid specific volume changes.

To .control pressure level at the inlet and outlet of all groups .to enable orderly transition from parallel oper ation to series operation without flow upset or disturbance, the present invention provides a pumping means between the outlet of one group and the inlet to another group in conjunction with directional flow control and throttling means as described below.

7 Referring more particularly to the drawings showing practical embodiments of the present invention:

FIG. 1 shows one form of my invention. Steam generator 1, denoted diagrammatically by the dot and dash outline, "includes heat absorbing preheating section 2,

generating sections Sand 3:; and superheating section 4, whichsections are connected by conduits 5c, 5, 5a, 6,

I 6aiand 6c.fDuring*no1 rnalfull load operation of the A storage container 23 having water level 23a.

steam generator, flow from the superheater 4 is through conduit 7, through flow. control valve 8. 0t turbine 9 which drives electric generator 10.

Exhauststeam from turbine 9' passes through conduit 11 to'condenser' 12 wherein the exhaust steam is condensed by means-of cooling water conduits 59 and collected in hotwell 13. :The collected condensate passes through conduit 14 to pump" 15 wherein the pressure is raised to the first working level. Pump 15 discharges through conduit 16 to' filter 'dimineralizer 17 wherein impurities are removed from the condensate. Flow continues through conduit 13 to low pressure heater system '19 and through conduit-"20 to deaerat'or 21.

Flow control fvalve" 22 regulatesfluid flow to deaerator 21.

After the condensate is I deaerated, it drops to water 7 Flow passes through conduit 24 to feedwater pump 25 which raises the fluid pressure to the working pressure in the steam points permits lower pressure drops and lower. velocities through these conduits during minimum flow operating generator. 'The,feedwater pump 25 ;discharges through conduit 26'to a high pressure feedwater'heater system 27 and back to the "steam generator through conduit 28 to the preheating section 2, generatingsections 3 and 3a 1 in parallel and superheating section4.

periods. This, in turn, permits greater water storagein" the generating section conduits for protecting the; tubes from unequal distribution of heat among the individual conduits. .Lower pressure drop through the conduits also prevents choking the flows in conduits which have a tendency to overheat. If the number of generating groups is increased above two,'the above eifects'will be section component conduit 7 During startup of the steam generator from the cold state, flow is'circulated from water storage container 23 through conduit 24, pump 25, conduit 2 6, heater- 27 and conduit 23 in series to the preheater 2, through conduits 5c, 5, valye 6Lgenerating section 3, conduit 29,.conduit 31 iand' check'valve 32-,conduit 33 to conduit 5a on the amplified. Thus, high minimum flows can be maintained in the individual conduits of the generating section which,

,' in. percentageof rated flow,'are' very substantially above the percentage of rated feedwater flow to the steam generat i r he n e i n- I Villie'rru'sing thev series arrangment shutdown and the. unitisbeing, heated to working level, or being cooled for maintenance, it isdesirable to minir nize temperature differentials between oramongth'e'gent eratingsection component conduits groups, to minimize differential expansions of, the parts. Thiscan be done by during 1 startup and downstream side ofl check valve 35. Positive closing device 36 isoperated to hold check valve 35 closed at this-time. -;Flow continues through generating section 3d,.conduit6a, conduit -60, conduit"36 ,and throttling valve SZ'Which regulates pressure in the generating sections 3 and 13a, to flash tank 38, through conduits 39 taiiier 23. I I

In thezfo'regoi-ng operation, pump 25 provides the work and) and flow control'valvefil to 'watefstorage coning pressure for the fluid. Pump, 50 between conduits 29 conservation of .heat in the steam generator startupisys tern and minimizingheatlosses to'a heat sump as 'a'con denser when the steam generator is below operating tem- This is described below. 7 a

' M a t rrb of Wi 1' shown) foryfiring fuel .inhburners 42. Burners" 42 .are

" fired rnakingfjheat' available to the heat absorption conduits 2,3,, 3a and 4;" As thefiuid'is heated in conduits heat" conduit: groups isequali The. pressure.drop-through demure generating section is. equal to the pressure drop through any one parallelicircuit. In series operation,

, th pressuredr'op through each' of the groups becomes and 33 isstopped. Check valv'es5j51 and 52 have positive-closingdevicesfitl" and 53, respectively, which are operated to hold check valves '51.and;52 closed at this time; Flowing-fluid through generating sections 3 and -3a fin series arrangement, without the use of pump 50 until the fluid heatsup, eliminates the additional horsepower-whichwould be required by thepu'mp'Sl) driver whenharidling' cold'fluid. V

known construction (not steam is formed in flash tank 38 and'fpasses through conduits 4 3 and 44 throughflow c'ontrolvalv'e 45 .to the steam space of; feedwaterheater'27,-heatingfup,the feedwater, enteringffrom conduit 26 1219361156 the saturation temperature of the flash tank steam. Water in the flash tank is drained to the water storage container 23 through conduits 39 and 40 and flow control valve 41. Valves 37, 45 and 49 have control means whereby the flash tank 38 is normally operated during startup at pressures whose saturation temperature is substantially above the designed operating temperature of condenser 12. Steam from flash tank 38 may be supplied to deaerator 21 through conduits 43 and 46 and flow control valve 47 to heat condensate entering the deaerator 21 through conduit 20 and flow control valve 22. Surplus steam may be discharged to condenser 12 through conduit 48 and flow control valve 49. Through such use of bypass system heat (flow through conduit 48 not included), the differential temperature between the feedwater entering the preheater 2 and the flow through conduit 36' can be maintained below 100 F. when heating up the unit. Differential temperature across the generating section will be even less.

When the eflluent flow through conduit 36' reaches 500 F. or some suitable temperature consistent with system design, the pump 50 may be started. Valve 51 is per mitted to open by withdrawing positive closing device 39.

Positive closing devices 36 and 53 are withdrawn so that fluid flow through valves 35 and 52 can pass freely in the direction of the arrows. Flow will, however, be prevented in the reverse direction. This provides the potential for unrestricted parallel flow through both of generating sections 3 and 3a from preheating section 2 to the inlet of the superheating section 4. The unit is designed for equal pressure drops when proportionate amounts of fluid are flowing through conduit 5, valve 69, generating section 3, check valve 52 and conduit 6 in parallel with conduit a, check valve 35, generating section 3a, and conduit 6a. During operation, positive closing devices 36 or 53 can be adjusted to throttle valves 35 or 52 to balance actual flows between circuits or to adjust for differences in heat absorption characteristics between sections 3 and 3a.

In principle, if the pressure drops across sections 3 and 3a with connecting conduits are equal for any given balanced ratio of flows between the sections, when operated in parallel, the same ratios will continue as parallel flow from preheating section 2 is increased or decreased.

However, when the generating sections 3 and 3a are operated in series, the fluid will be heated continuously throughout the series circuit. The fluid temperatures, specific volumes and enthalpies will be higher in the second section 3a. Therefore, for equivalent mass flows through sections 3 and 3a, pressure drops will be lower in section 3 than they are in section 3a. Compensation must be made for this factor in order to gradu ally and smoothly transfer from series to parallel flow as the output of the steam generator increases. Compensation is in the form of throttling control in valve 60 in conduit 5. Valve 60 is throttled automatically to maintain pressure drop and flow through the section 3 circuit, from the intersection of conduits 5 and 5a to the intersection of conduits 6 and 6a, approximately equal to the pressure drop and flow through the section 3a circuit and between the same intersection points.

FIG. 1a shows the valve 60 control system in detail and is described below. Valve 60 could alternately be located in any point of the section 3 circuit upstream of conduit 29.

When there is no flow through pump 5%) to section 3a and when valves 35 and 52 are free to pass fluid in the downstream direction, flow from section 2 is shared by sections 3 and 3a in a way which causes equal pressure drop through the two sections from conduit 50 to conduit 60. Parallel operation of sections 3 and 3a results. Pressure drops through sections 3 and 3a are in excess of the pumping head across the pump 51) circuit at this time.

Pump 50 establishes a control pressure differential from the junction of conduits 6 and 29 to the junction of conduits 5a and 33. The latter junction is the highest in pressure. When the control pressure dilferential across the pump circuit is greater than the fluid flow pressure drop across generating section 30, fluid pressure on the downstream side of valve before it enters section 3a is higher than the pressure on the upstream side of valve 35. Thus, valve 35 is held in the shut position. Also, the pressure on the downstream side of valve 52 will be greater than the pressure on the upstream side and the valve will be held closed. Thus, flow from preheating section 2 will flow serially through section 3, the pump circuit and section 3:: to conduit as long as the pressure rise across the pump 50 circuit is greater than the pressure drop through section 3a and connecting conduits.

Since pressure drop is related to flow, the pumping head determines the flow quantity at which the transition from series flow to parallel flow will start to take place. The effective control head characteristics for various fiows between conduits 6 and 5a can be adjusted by throttling valve 51 as required to optimize the transition point and rate of transfer with rate of load change. Closing valve 51 lowers the flow rate at which transition takes place. Also, it will decrease the rate of transfer with respect to rate of load change. Depending upon the exact pressure drop characteristics of sections 3 and 3a with connecting conduits and the flow ratios established for each, there may be some partial flow through either valve 35 or valve 52 for the series arrangement at minimum flow.

As diiferential pressure across and flow through sec tion 3a and connecting conduits increase to equal and exceed the differential pressure across the pump 50 cir cuit, the pressure drop across check valves 35 and 52 Will equalize and flow from preheating section 2 toconduits 5 and 5a and flow from sections 3 and 3a direct to the superheating section will move in the direction of more parallel flow. The flow through section 3 will be direct from section 2. The flow through section 3a will be a combination of the diminishing flow from the discharge of section 3 mixed with increasing flow direct from section 2 through valve 35. The flow through valve 52 will be the flow through section 3 less the diminishing flow through the pump 50 circuit. It should be noted that no portion of the fluid flow is recirculated through any of the conduits within the generating section in the series-parallel arrangement. In the series arrangement, the flow passes through both sections 3 and 3a sequentially. As the transition is made from series to parallel once-through flow, part of the flow from section 2 bypasses section 3 gradually up to the point where flow from section 2 is proportionately shared by sections 3 and 3a at full load. The same is true at the outlet of section 3. Flow from section 3 gradually bypasses section 3:: up to the point where all the fiow discharges from sections 3 and 3a direct to section 4 for the parallel full load arrangement.

As the pressure drop through each of sections 3 and 3a increases above the maximum pumping head of the pump 50 circuit, flow through the pump 50 circuit from conduit s to 5a will cease. At this time the generating sections 3 and 3a are operating in parallel.

The pump 50 is of the centrifugal type and its total dynamic head characteristics preferably should rise continuously as flow through the pump is decreased. However, if the pump has different characteristics, they can be shaped properly by the throttling of valve 51. The pump 50 can be (but not necessarily) driven by a canned type electric motor which doesnot require any. high pressure seal for the driving shaft. The pump impeller and motor rotor are all submersed in the pumped fluid medium. Conduit 54 permits some small amount of cooling fluid flow to pass through the pump when it is can be calibrated.

1 *The. shaftimoyement is a operating and its pumping head is less than the fluid flow pressure drop across 3a and connecting conduits. Conduit 54 discharges to conduit 6a or some other point in conduit 6 at the discharge from the junction of conduits 6 and 29. Flow control valve 55 is located in conduit 54 to regulate the amount of cooling fluid flow. Other valve structures or combinations may be used for elements 32,35, 51, 52 and 60 which accomplish the same functions. I

As flow through the steam generator conduits is decreased bclowfull load, the reverse operation takes place. Parallel flow through generating sections 3 and 3a is converted to series flow as the discharge flow from section 2 is decreased. When shutting down the steam generator, pump 59 can continue in operation until cool I fluid would overload its driver, at which time positive closing devices 30, 36 and 53' are operated to hold valves I 51, 35 and 52 closed and pump 50 is shut down.

Orifices can be installed in conduits 5 or 6, 5a or 6a,

29 or 33 also for balancing flows in the sections 3 and 3a and pump 50 circuits.

It, should be noted that during startup, when sections 3 and 3a are in series annangernent, all of the flow entering the steam generator through conduit 28 passes serially from the inlet of section 2 to conduit 6c in multiple passes without recirculation and may be drawn off through conduit 36 to the flash tank 38. Water drains from the flash tank 38 may be discharged to condenser 12 through conduits 39 and 56. Flow'control valve 57 regulates flash tank drainflow through conduit 56 to maintainflash tank water level 58 constant. Thus, all of the 'fluid circulated intthegenerating sections passes.

to the flash tank. lmpurities'in 'the conduit 36.flow';

are contained in the flash tank water; The flash tank water may be drained to condenser 12 forycooling after which it may .be' passed through purificationequipment 17 and returned to the cycle through deaerator 21. This 1 facilitates rapid cycle cleanup contrasted to the case.

where flow is continuously recirculated within the generating section. In the latter case, removalof impurities from. the generating section'fluid is a matter of dilution if the flow through a bypass as conduit 36' at the discharge of a generating section recirculation loop is below, the flow rate in the loop.

Minimum flow to the steam, generator through conduit 28 duringstartup, is notjobjectionableas it is desirable to draw'off flow from the generating section .through c'onduit36' for fluid purification.

.FIG. 1a illustrates the control system 'associated'with valve 60, as describedfor FIG. .1. FIG. la'shows the steam generator segmentof FIG. 1. Taps'1ti9 and 111 are locatedin conduits 5a and 6a. Conduits 111 and 112 conduct fluid from these points to difierential pressure unit 113. Differential pressure unit 1 13 may be of the: type manufactured by the Barton Instrument Corporation;

1 a FIG. 1

chambers .114 and 115 increases.

tem. Thus, the movement of the torque rod is proportionate to the pressure difierential between chambers 114 :and 115. Shaft 120 connects to transmitter 121.which .converts movement of shaft 120 tea 3 to 27 p.s.i.g. air signal increasing as the differential pressure between The 3 to 2 7 .p.s.i.g. range varies proportionally with the differential pressure vrange of unit113. Conduit 122 supplies 30 p.s.i.g. air to transmitter .121v for ,conversion'to the 3 to 27 p.s.'i.g. differential pressure relative output signal in conduit 123.

Taps 124 and 125 are located in'conduits 5 and 6. Conduits 126 and 1217' conduct fluid from these points to differential pressure unit 113' which is the same as 113. Shaft 120 is the same as shaft 120, and transmitter 121 is the same as transmitter 121. Transmitter 121 sends a 3 to 27 p.s.i.g. signal .to conduit 128 which is proportional to pressure diiferential in 113. Conduits 123 and 128 feed to computing relay'129 of well known type, such as that manufactured by the Bailey Meter Company, Cleveland, Ohio and shown in Fig. 10 of their product specification P99-3, copyright 1956, Form No. CPr99-3D, printed March 1960. Computing relay 129 has proportional plus reset action and its formula is: A-D=A'C'+M- J where p a g a AA=change in A pressure AB=change in B pressure AC =change in C pressure AD=change in D pressure R=percent proportional band/10 0' pressure across steam generating section 3a is greater than the differential pressure across the generating section 3, A pressure in conduit 123 will be higher than the .B pressure in conduit 128 and thus the D pressure willincrease. T he D pressure'is transmitted from relay 129 to conduit 130. As pressure rises in conduit 130, valve 60 will close 'until differentialpressures across the 3 and 3a sections are equal. If the pressure in conduit 123 is less than in conduit 128, the D. pressure in conduit 130 will decrease and valve 60 will open until the pressures in conduits 123 and 128 are equal. Relay 129 is equipped with calibration springs which permit adjustment ofthe equilibrium pressures between conduits123 and 128. Thus, conduit 128 pressure can be controlled slightly above or below the conduit 123 pressure to'compen'sate for variable resistance as flow increases throughvalves 35'or 52, or to change thefiow ratios between generating sections 3 Monterey Park, California, :Model. 224 consisting of two opposed chambers 114, and' connected by a spacer piece-1 16. Conduit 111;. connects tocharnber 114 and conduit 112 connects to the chamber 115. In each cham ber is a sealed bellowsl'l'l and 118 filled inside with a: liquid. The liquid system cross' connects the twobellows. 7

One end of the bellows is. attached to the chamber. ata point-which opposes the other chamberandseparates the.

portions of the chambers Whichare'around the outside of the bellows from each other} The bellows, are cross'fcon nected atthe other end or far endrfrom each other by means of'a shaft 119 which runs through-the spacer'piece.

Thus, .when a differential pressure'exists between the chambers 114 and 115,"'the shaftis moved'in the: directionof the chamber having .thelov'ledpressiirj; 'The bellowsjtar'e equipped withrange springs (not sho'wn) to Thusshaf t.119movement resist movement ofthef'shaft.

, convertedto. torque in ro d in the spacer piece. and which is external to thevfiuidsys- Conduit feeds, to positioning relay 131 of well knowntype, such as that manufactured by the Bailey Meter Co.',; Cleveland, Ohio and as shownvin their -product f specification P995, copyright 1957, Form 'No. CP 99-5A, v printed April 1960. Positioning relay 131 is equipped with a mechanicalflink 132 which is connected to the operator-stem of valve160 and which indicates the open position'of ,va1v'e60. Mechanical link 132 rotates a cam finf relay 131 which, in turn,applies.spring tension on a balance beam. The balance beam converts: air pressure in conduit 130to valve operator 133 working pressure in conduit 134; ;The cam can be shaped so' that the valve 60 will have any given opening'forfany given pressure in conduit 130.; Thus, pressure drop characteristics in generating'sec tions i'i and 3d can be" paralleled with valve 60 opening. The valve operato'r'j133 is of thediaphragm or piston type and is soarrangedi that.increasing air pressure i'n conduit 134 closes.valve,60,,and decreasing air lgpressure-in conduit"1'34 opens valvef 60.. Conduit 135 is a. 50 p.s,i.g. air supply toirelay 131 to, supply working air I I for-conversion in the relay to -the.signal in conduit 134.

.- Va lve is-provided with a stop (not shown.) which -pre-,

vents valve closure beyond a minimum limit. Conduit 123 also feeds to pressure switch 137. Rising pressure in conduit 123 closes a contact (not shown) in circuit 138 as it passes through switch 137 to energize electric solenoid 139 which actuates three way valve 140 so that the A chamber of relay 129 vents through conduit 141, through needle valve 14-2 to atmosphere. This prevents valve 60 from unbalancing flow between sections 3 and 3a as flow increases through section 3a during and/or after transfer from series to parallel flow. Venting the A chamber of element 129 causes valve 60 to open.

FIG. 2 illustrates how the series-parallel arrangement may be used for multiple generating section component conduit groups. Correspondingly numbered elements in FIG. 2 are the same as those in FIG. 1 and the operating principles are the same. Elements 29 through 33 inclusive, 35', 36', 50' through 55' inclusive and 60' are equivalent to their counterpart elements without the prime designations. In the case of the multiple seriesparallel arrangement, required minimum flow through conduit 28 to section 2 during startup is reduced as the number of series-parallel groups are increased, or con versely, the minimum flow through each of the seriesparallel groups is increased if the flow through conduit 28 is held constant.

FIG. 2a illustrates the control system associated withvalves 60 and 60' as described in FIG. 2. FIG. 2a shows the steam generator segment of FIG. 2. The description and elements in FIG. 2a parallel those for FIG. 1a. Only the differences are described below. The pressure drops through steam generator sections 3 and 3a are each compared with the pressure drop through section 3b. Each is corrected and individually biased against section 3b.

Elements 124' to 134' and 139 to 142' of FIG. 2a correspond to elements 124 to 134 and 139 to 142, respectively, of FIG. 1a; and elements 113 and 121" of FIG. 2a correspond to elements 113 and 121', respectively, of FIG. 2a.

FIG. 3 is a plot of water-steam characteristics at 3500 p.s.i.a. and shows fluid temperature and fluid specific volume plotted against fluid enthalpy (B.t.u./lb.). The fluid enthalpy through sections 2, 3 and 3a, and 4 gradually increases as fluid flows through the steam generator 1 when burners 42 are fired. Enthalpy rise is a function of heat input to the heat absorption conduits.

For a steam generatorv designed to operate in the 3500 p.s.i.a. range as shown on FIG. 3, it could be expected that the generating section enthalpy range would be from 500 to 1000 B.t.u./lb. during the startup condition (when there was substantial enthalpy rise through the unit after initial warrnup) and from 630 :to 1100 Btu/lb. during normal operation above 50 percent of rating. Where the individual conduits of both generating section conduit groups 3 and 3a in FIG. 1 are in close structural association, one with another, temperature differentials between conduits must be considered. There will generally be no problem if all the conduits are operated in parallel and they are also structurally arranged in parallel. If,

however, an individual conduit of section 3 is in close structural arrangement with an individual conduit of section 3a and the groups are operated in series when the inlet fluid temperatures to groups 3 and 3a are for eX- ample 510 F. and 670 F respectively, there can exist a metal temperature different of 160 F. at points in the close structural association. Also, from FIG. 3 it is noted, that, during series operation of sections 3 and 3a where group 3 inlet temperature is 510 F. and group 3a inlet and outlet temperatures are 670 F. and 720 F. respectively, the specific volume of the fluid is below 0.03 cu. ft./lb. for group 3 and ranges from 0.03 to 0.075 cu. ft./lb. for group 3a. Thus, it can be seen that pressure drops will be different for the same fluid mass flow in each section at this time.

Where such factors are of significance in the structural design of a steam generator and to minimize temperature differentials between individual conduits of multiple conduit groups and to more nearly equalize pressure drops through the parallel conduit groups in the transition from series to parallel arrangement, the modified invention shown in FIG. 4 has been devised. Operation for FIG. 4 is similar to that described for FIG. 1 above. Correspondingly numbered elements have similar functions.

In FIG. 4, steam generator 1' includes heat absorbing preheating section 2, generating sections 3' and 3" in parallel with section 3a, and superheating section 4, which sections are connected by conduits 5c, 5 and 5a, 6, 6 and 6a, and 60. It should be noted that generating section 3 of FIG. 1 has been divided into two sections 3' and 3" in FIG. 4. When the flow is arranged in parallel, flow through sections 3' and 3" in series is in parallel with flow through section 3a. When the flow is arranged in series, flow is serially through sections 3', 3a and 3".

During normal full load operation of the steam generator, flow from the superheater 4 is through conduit 7, and through flow control valve 8 to turbine 9 which drives electric generator 10. Exhaust steam from turbine 9 passes through conduit 11 to condenser 12 wherein the exhaust steam is condensed by means of cooling water conduits 59 and collected in hotwell 13. The collected condensate passes through conduit 14 and pump 15 wherein the fluid pressure is raised to the first working level. Pump 15 discharges through conduit 16 to filterdemineralizer 17 wherein impurities are removed from the condensate. Flow continues through conduit 18 to low pressure heater system 19 and through conduit 20 to deaerator 21. Flow control valve 22 regulates fluid flow to deaerator 21. After the condensate is deaerated, it drops to water storage container 23 having water level 23a. Flow passes through conduit 24 to feedwater pump 25 which raises the fluid pressure to the working pressure in the steam generator. The feedwater pump 25 discharges through conduit 26 to a high pressure feedwater heater system 27 and back to the steam generator 1 through conduit 28 to the preheating section 2. From section 2, flow passes to generating sections 3' and 3a in parallel. Flow from section 3' discharges to 3" and flow from 3" and 3a discharge to the superheater 4.

During startup of the steam generator from the cold state, flow is circulated from water storage container 23, through conduit 24, pump 25, conduit 26, heater system 27 and conduit 28 in series to the preheater 2, conduits 5c, 5 and valve 60, through generating section 3, conduit 29, conduit 31 and check valve 32, conduit 33 to conduit 5a on the downstream side of check valve 35. Positive closing device 36 holds check valve 35 closed at this time. Flow continues through generating section 3a, conduit 6a, conduit 29, conduit 31' and check valve 32, through conduit 33' to conduit 6. Positive closing devices 53 and 53 are operated to hold check valves 52 and 52' closed at this time. Flow continues through section 3", and conduits 6', 6c, 36 to flash tank 38. Throttling valve 37 regulates pressure in the generating sections. Flow continues through conduits 39 and 40 and flow control valve 41 to water storage container 23., In the foregoing operation, pump 25 provides the working pressure for the fluid. Pumps 50 and 50', between conduits 29 and 33 and between conduits 29' and 33', are stopped.

Means are provided (not shown) for firing fuel in burners 42. Burners 42 are fired making heat available to the heat absorption conduits 2, 3, 3" and 3a and 4. As the fluid is heated and passes through valve 37 to the flash tank 38, steam is formed in flash tank 38 and passes through conduits 43 and 44 through flow control valve 45 to the steam space of feeclwater heater 27, heating up the feedwater entering from conduit 26 close to the saturation temperature of the flash tank steam. Water in the flash tank is drained to the'water storage container 23 v (2) As long'as the pressure dropthrough i 1 1 through conduits 39 and 40 Valves 37, 45 and 49'have control means whereby the flash tank 38 is normally operatedduring startup at pressures whose saturation temperature is substantially above the designed operating temperature of condenser. 12.

Steam from'flash tank 38 may be supplied to deaerator 21 through conduits 43 and 46 and flow control valve 47 to heat the condensate entering the deaerator 21 through conduit 20 and flow control valve 22. Surplus steam may be discharged to condenser 12 through conduit 48 and flow control valve 49.

When the eflluent fiow F. or some suitable temperature consistent with system design, pumps 50 and 50' may be started. I Valves51 and 51 are permitted to open by withdrawing positive closing devices 30 and 30'. Positive closing devices 36, 53 and 53' are withdrawn so that fluid fiowlthrough valves ss, 52

and flow control valve 41.. V

through conduit 36 reaches 500 and 52' can pass freely in the direction of the arrows.p

Flow will, however, be prevented in the reverse direction. This provides the potential for unrestricted parallel flow through the generating section from preheating section 2 to the inlet of the superheating section 4. The flow char acteristics with respect to series and parallel arrangements and the combinations in between are similar to 'FIGS. 1, and 4.. However, the total pressure drop across sections 3' and 3" will more nearly equal the pressure drop across section 3a. Fluid specific volumes are low in section3' and highin section 3" in the series arrangement (flow through sections 3', 3a and 3." in series). Fluid specificvolume in section 3a is moderate. There- 7 fore, there is less compensation required in valve 60 when transferring from series to parallel flow. The division of heat absorption surface between sections 3' and 3"'will determine the amount of compensation required in valve 60 as Well as the location of valve 60. 'If during the transition period from series to parallel flow, the pressure dropthrough the section 311 circuit is lower than the combined pressure drop throughthe sections 3' and 3'! circuit, then valve 60 should be located in conduit 5a or 6a downstream of the junction with conduit 33' and up the superheater 4, -the following is one approximate combination of conditions possible: 510 F. at section 3 inlet, 580 F. at section 3 outlet and section 3a inlet, 700 F. at section 3a outlet and section 3 inlet and, 720 F. at section 3"- outlet. If the sections 3' and 3a inlets are in close structural association there will be a temperature' diiierential on only 70 F. If the section 3' outlet and section 3" inlet are in close structural association with an intermediate point'of section 3a, there will be temperature differentials of about 55 to 65 F. If the section 3" outlet is-in close structural association with the section 3a outlet, there will be a emperature differential of about 20 to F. Therefore,

temperature differentials can be distributed to minimize diiferential structural expansions. Such structural association isshown .on FIG. '5'. I

-With respect to specific volumes it can be seen on FIG. 3that for the above mentioned temperatures, average specificvolumes are about .22cu -ft./lb. in section 3', about stream of the junction with conduit 29 instead of in conduit 5, as shown, or alternate location inconduit 6 upstream of the junction with conduit 29. This determination will be made to suit the specific steam generator design in question and is inconsequential to this invention. Valve 60 merely balances resistance in' the circuit including sections 3' and 3'. withresistance of the circuit ineluding section 3a from conduit Soto 6c. Instead of re -v locating valve 60, it'is possible to adjust jpre'ssure differentials in the circuits by means of-orifices. f

Relationships .for valves 35,v 52 and 52' areas follows:

(1) .As long asthe pressure drop through the section 3' I circuit is less than thepressure rise through the pump v circuit, valve 52' will be closed.

.52 cu. ft./lb. in section 3" and, in about .35 cu. ft./lb. in section 3a. Fluid specific volumes in the combined sections 3' and 37 tends to average with the fluid specific volumes in section 3a.

' Thus, a balance in specific volumes and pressure drops as) well as temperatures can be achieved in the generating section when transferring from the series to parallel arrangement, with minimum control action required from valve 60. In some cases valve can be omitted.

FIG. 5 is a diagrammatic arrangement of one configuration of the steam generator shownin FIG. 4. Other arrangements are possible. Feeclwater' flow enters the steam generator 1 through'conduit 28 and flows to preheating section 2, through section 2 to conduits 5c and 5, valve 60 and inlet header 61 supplyingfluid to generating section 3'. Generating section'3' consists of many parallel tubes which, when integrated with generating section 3a, form a circumferential waterwallfor the lower portion of'combustion furnace 62. Generating section 3' flow discharges tooutletheader 63 and toconduit 6 wherein there is valve-52. Conduit 6 discharges flow to-theinlet header 64 supplying fiuid togenerating-section 3". As

in .the case ofgenerating section 3', generating section 3" also consists of manyvparallel tubes which when integrated with generating section 3a form a circumferential waterwall'for the upperportion of combustion furnace 62. Generating sectionS flow discharges to outlet-header system 65 which feeds to the downstream superheating .section 4. Part ofthegenerating section in this case is not inj cluded in the series-parallel:arrangement.

the 'section'3u circuit .isless than thepressure rise across thezcorn-v bined pump 50 and 50' circuits, valve 52 will stay closed.'.

(3) As long asthe pressure drop through the section ,3"

. circuit is less than the pressure rise across the pump 50 circuit, valve 35 will stay closed.

For. other conditions, the said valves'will open. There} fore, in series arrangement flow .will'be, through sections 3, '3aand 3 in series. As the transition toparallel are] rangement-occur's flow will start to pass through valves 35,52 and 52 andwillincrease as fio'ws through pumps- '50 and 50' decrease. ,3 Minimuni flow for cooling pumps 50 and, 50' can be passed through conduits 5-4 'andi54' beingthrottled byivalves" 55'andv 55'. .The pumps, 50"and are'asidesclibed as for P 0 05 11 1 3 5 t'o pump Strand valve 32in parallel; Canned motor 71 is true of other features.

'67 supplying fluid to generating section'iia.

Conduit Be at the outlet of the preheating section 2 also discharges to; conduit 5a, through valve-35, to inlet header I Generating sectionfSa consists of many parallel tubes which are integrated with and interspaced between the many parallel tubes of generating'se'ctions 3' and. 3".v The section 3a discharges" to outlet'header 68; Since the section3z tubes are the full length of the furnac'efrom; header 68 to I header 67, the furnace walls can be supported from head- 7 c 68, the sections 3' and 3" being supported by section to conduit 60.

13a-" Ha'ngers69 connect header 6% to "supporting steel '70above the steam generator. Thus the arrangement of "FIG. 4 is developed in FIG.{5 permits construction ofan Y economical. furnace which can beeasily supported. Header 68 discharges to conduit 'a, valve52 and conduit a The conduit 29 connects to conduit, 6-and discharges drivegpurnpSt). Pump '5tiidischargesithrough valve 51 Ito conduit 33 and from thence to conduit 5a downstream of valve 35.

The conduit 29 connects to conduit 6a and discharges to pump 50 and valve 32 in parallel. Canned motor 71' drives pump 50'. Pump 50' discharges through valve 51' to conduit 33' and from thence to conduit 6 downstream of valve 52. e

The header 66 discharges to the multiple tubes which form the roof 72 of the steam generator furnace 62 and convection pass 73. The roof tubes 72 discharge to header 74 and from there fluid passes through conduit 75 to headers 76 which are interconnected through conduit 76a. Headers 76 supply fluid to the convection section wall enclosure 77 which consists of multiple parallel tubes. The discharge from wall enclosure 77 is collected in headers 78 and flows through conduits 79 to header 80. Header 80 discharges through conduit 81 to primary superheater inlet header 82 which feeds fluid to the primary superhaeter 83. Primary superheater 83 is disposed across the width of the furnace in multiple parallel circuits closely spaced and in a general configuration as is shown on FIG. for each parallel circuit. Primary superheater 83 discharges through conduits 84 to header 84a, connecting circuits 84b to header 85 which distributes fluid to radiant superheater 86. Superheater 86 is arranged in multiple pendant platens widely spaced across the width of the furnace. Superheater 86 discharges to outlet header 86a, through conduit 87 to header 88, and to final superheater 89. Superheater 89 is arranged similarly to superheater 86 except the elements are closely spaced in 89. Superheater 89 discharges to header 90. Header 90 connects to conduit 7 fluid then passes through governor valve 8 which supplies steam to the high pressure turbine 9a.

Turbine 9 of FIG. 4 is divided into high pressure turbine 9a, intermediate pressure turbine 9b and low pressure turbine 90 in FIG. 5. Steam from turbine 9a discharges through conduit 91 to heater 92 to low temperature reheater 93 arranged similar to superheater 83. Reheater 93 discharges to header 94 and conduit 95 to the inlet header 96 supplying fluid to the high temperature reheater 97 arranged similar to superheater 89. Reheater 97 discharges to header 98 and conduit 99. Interceptor valve 190 in conduit 99 regulates steam fiow to turbine 9b in times of emergency. Turbine 9b discharges to turbine 96 through conduit 161. Turbine 9c discharges through conduits 11 and 11' to condenser 12.

Burners 42 fire fuel in furnace 62 after mixing with air entering from windboxes 102. The air supply to the windboxes is not shown. The combustion products flow up through furnace 62 and pass over the heating surfaces 86, 89, 97, 93, 83 and 2 in series and discharge from the convection pass 73 through conduit 103 to an air preheater (not shown) where the hot gasses exchange heat with air used for combustion in furnace 62.

The furnace is so constructed that the wall tube conduits form an opening 104 at the bottom of the furnace. Combustion ash falling to the furnace bottom passes through this hole'to ash collection hopper 105.

Generator is driven by turbine 9a, 9b, 90 through connecting shaft 106.

The steam generator insulation and outer casing 197 covers the outside wall tubes of the furnace 62 and convection pass 73.

FIGS. 5a, b and 0 show details of the furnace wall construction of FIG. 5 in the vicinity of headers 63 and 64. The same numbers are used to identify the same elements. The individual tubes of each generating section are shown in the manner in which they connect to headers 63 and 64. The tubes are in close structural arrangement and are held together by the metal membrane 108 between the tubes and which membrane is welded to the metal tubes so as to form a gas tight enclosure.

FIG. 5:1 is a substitute. arrangement for FIG. 5a. FIGS. 5e and 5 supplement FIG. 5d. FIGS. 5d, e and 1'' show a detail of the furnace waterwalls comprising tube conduits from generating sections 3', 3" and 3a. The furnace wall section below headers 63 and 64 is essentially the same as shown for FIGS. 5a, b and 0. However, on FIGS. 5d, e and f, the tubes have been enlarged above headers 63 and 64. In doing this the membrane 108 is eliminated and the upper tubes are arranged so as to be tangent to each other. A skin casing 136 is arranged over the tube surface on the outside of the furnace to make a gas tight enclosure over which steam generator insulation and outer casing 107 is applied.

A control system for valve 60 associated with generating section series-parallel conduit flow shown in simplified form in FIG. 4 and as arranged in FIG. 5 is shown in FIG. 5. The description of operation is the same as that for FIG. 1a. Elements numbered 109 to 113, inclusive, 1.13, 121, 121, 122 through 135, inclusive, 137 through 142, inclusive, are shown on FIG. 5. They correspond to the same numbered elements as shown on FIG. 1a.

Differential pressure unit 143 is similar to element 113 except it opens an electric switch in unit 147 when there is approximately zero pressure diflerential across valve 52 as transmitted to unit 143 through conduits 145 and 146 or when the pressure upstream of valve 52 is higher than the downstream pressure. This opens an electric circuit 144 which permits solenoid 139 to be de-energized so that control action can be applied to valve 60.

FIG. 6 is a modification of the system shown in FIGS. 2 and 4 and which has been simplified to show the basic series-parallel circuits only. The pump 50, 50' and 50" bypass and cooling circuits have not been shown for the sake of simplicity.

In the above descriptions it has been shown how flow through the series-parallel circuits may be controlled in series or alternately in parallel either with or without the use of a pump between sections in the series arrangement. An arrangement of a modified invention is shown on FIG- URE 7 wherein the pumps are omitted and the seriesparallel flow is controlled through the use of valves only.

Element numbers on FIGURE 7 are as previously described. Valves 35a, 52a and 52b are substituted for valves 35, 52 and 52 as shown on FIGURE 4 respectively. When flow in the circuits is in the parallel arrangement, valves 35a, 52a and 5219 are open. When flow in the circuits is in'the series arrangement, valves 35a, 52a and 52b are closed. In order to transfer from series to parallel arrangement valves 35a, 52a and 52b are opened simultaneously and gradually to balance flow among the respective circuits as described above.

One possible control system arrangement is shown on FIGURE 7. The rules for transfer for FIG. 7 are as follows referenced to the pumps 50 and 51 of FIG. 4.

Valve 52]) replaces pump 59'. Valve 52]) is controlled in the shut position until pressure drop across the :9 section and connecting circuits equals a fixed value which is equivalent to the hydraulic head across pump 50' at which time valve 52b opens to maintain a preselected ditferential pressure across the 3" section and connecting circuits.

The valve 52:: replaces pump 50. Valve 52a is controlled in the shut position until pressure drop across the 3:2 section and connecting circuits equals a fixed value which is equivalent to the hydraulic head across pumps 50 and 54) at which time valve 52a opens to maintain a preselected differential pressure across the Sn section and connecting circuits.

The valve 35a replaces valve 60. Valve 35a is controlled in the shut position until pressure drop across the Sn section and connecting circuits equals a fixed value which is equivalent to the hydraulic head across pump 50 at which time valve 35:: opens to maintain a preselected differential pressure across the 3a section and connecting circuits.

The control of valves 35a, 52a and 52b is similar. On FIG. 7 the control for valve 52a only is shown. Pressure differential across points 1'99 and is sensed in pressure difierential unit 113. The pressure differential is transformed to a proportional variable control air signal in transmitter 121 which is conveyed through conduit 128 and modifications may be ;-following claims. 7 1 claim: r

' t l. A fluid circulation which in this case connects to the B chamber of relay 129. The control air Signal in conduit 128 increases as the pressure differential across section 3a increases.

A pressurized air supply in conduit 150 is reduced in pressure through pressure'regulating valve 149 to a preselected value and connects to the A chamber of relay,

129 through conduit 148. The preselected pressure in conduit 148 furnishes the set point for relay 129. Spring loading adjustments in relay 129 may replace the air signal in conduit 148. Alternatively, the pressure in conduit 148 may be programmed with load through the use of a function generator (not shown) tofvary the setpoint in chamber A. If the B pressure is lower than the A pressure, pressure in conduit 1311 increases and closesvalve 52a (which functions like valve 61) as described for FIG.

16 duit means serially connecting all of said elements, at least a portion of said'steam generating section comprising multiple parallel conduits arranged in multiple conduit groups, conduit and fluid flow control means to selectively flow fluid from said preheating section'through'the conduit groups in series or parallel arrangement, or in combinations'therebetween, before said fluid enters at least a portion of said downstream superheating section, said conduit and fluid flow control means being arranged to flow fluid in the conduit groups more serially as steam generator load is decreased, and more parallelly as steam generator load is increased, throughout at least a portion A of the load range of the steam generator.

la). Thus, when flow through section 3a produces a'presy sure drop which results in B pressure exceeding the A pressure in relay 129, valve 520 will open as aresult or decreased pressure in conduit 130. Thus, as flow through section 3a is increased as a result of increased flow through conduit 28, flow is! passed more parallelly through sections 3' and 3a from section 2, and from sections 3 and 3a to section 4; The controls for valves35a and 52b are'identical to those for valves 52a, except for A pressure set point and location of pressure taps; Taps 151 and 152 may be used for control of valve 35a. Taps 153 and 154 may be used for control of valve 52b. Thus, it will be seen that I have provided cfiicient systems for improving the fluid circulation of a once-through type steam generator during startup, shutdown and sustained partial load operation, including a novel apparatus and system for flowing fluid serially, parallelly, and in combination thereof, in an intermediate section of the steam generator heat absorption conduits; furthermore, I t

have provided a novel apparatus and system for accomplishing the above by means of a'pumping system and directional flow control and throttling control means interconnecting and within the steam generator conduits; also, I have provided a novel apparatus and system for operating the steam generator intermediate heat absorption, series-parallel conduits in series arrangement during start-' up through the use of flow control valves; also, I have 7 provided a novel structural arrangement for the steam generator. furnace walls wherein metal temperature differentials among component fluid conduits inclose structural association is minimized and'wherein pressure drops through the series-parallel fluid circuits for flow control purposes is more nearly equalized throughout the operating load range.

' While I have illustrated arididescribed several embodi -ments of my inventionyitwill be understood that these 1 7 are by way of illustration only, and that various changes made-within the scope of "the 7 system fora once-through type steam generator having elements comprising a feedwater inlet, a preheating section, a generating section,'a' superheating section, a steamsoutlet from the superheating sec- 7 'tion for furnishing steam to a steam consumer, fluid con- 2. A fluid circulation system for a once-through type steam generator as recited in claim 1, and where said steam 'fgenerator is designed'for supercritical pressure operation, including pumping and conduit means for raising fluid "pressure intermediately between the conduit groups when flowing fluid serially therethrough. I

3. A fluid circulation system for a once-through type steam generatoras recited in claim 1, including means to selectively throttle fluid flow in any one of the said multi 'ple conduit groups to'proporti'on flow in that multiple conduit group with respect to fflow in another multiple conduit group. a

*4. A fluid circulation system for a once-through type steam generator as recited in claim 1, including a combustion furnace having a wallenclosure at least a portion of which comprises said multiple parallel conduits of said generating section, said individual conduits of one conduit group being arranged intermediately between the individual conduits of another conduit group. I

5. A fluid circulation system for a once-through type steam generator as recited in claim 1, including a combustion'furnace having a wall enclosure at least a portion "of which comprises said multiple parallel conduits of said multiple conduit group and the inlet of said'third multiple conduit group: I .i

References Cited in the iile of this patent UNITED STATES PATENTS [2,900,792 Buri Aug. 25, 1959 2,989,038 Schwarz June 20, 1961 3,038,453 Armacost June 12, 1962 FOREIGN ,P'ATENTS, I ,t j

I 741,701 Great Britain" Dec, 7, 1955 768,201 Great Britain Feb.'13, 1957 G ea Brita i $01,0 "P -asses Sep 1 5 

1. A FLUID CIRCULATION SYSTEM FOR A ONCE-THROUGH TYPE STEAM GENERATOR HAVING ELEMENTS COMPRISING A FEEDWATER INLET, A PREHEATING SECTION, A GENERATING SECTION, A SUPERHEATING SECTION, A STEAM OUTLET FROM THE SUPERHEATING SECTION FOR FURNISHING STEAM TO A STEAM CONSUMER, FLUID CONDUIT MEANS SERIALLY CONNECTING ALL OF SAID ELEMENTS, AT LEAST A PORTION OF SAID STEAM GENERATING SECTION COMPRISING MULTIPLE PARALLEL CONDUITS ARRANGED IN MULTIPLE CONDUIT GROUPS, CONDUIT AND FLUID FLOW CONTROL MEANS TO SELECTIVELY FLOW FLUID FROM SAID PREHEATING SECTION THROUGH THE CONDUIT GROUPS IN SERIES OR PARALLEL ARRANGEMENT, OR IN COMBINATIONS THEREBETWEEN, BEFORE SAID FLUID ENTERS AT LEAST A PORTION OF SAID DOWNSTREAM SUPERHEATING SECTION, SAID CONDUIT AND FLUID FLOW CONTROL MEANS BEING ARRANGED TO FLOW FLUID IN THE CONDUIT GROUPS MORE SERIALLY AS STEAM GENER- 